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-Traces:

Conductors on cell surface collect electrons from N-layer and distribute electrons to P-layer

Electron Flow:

Through circuit, from N-layer to P-layer 0-

Free Electrons:

Pile up in N-layer and can only move to P-layer through circuit

Load:

Electrons passing through circuit do work

Free Electrons:

In P-layer can be bumped across P/N junction by sunlight, attracting more electrons through circuit

Holes

(Si) Silicon Atom: 4 electrons in outer shell. Shares with other silicon atoms to form a stable crystal bond of 8 electrons.

® Boron Atom: 3 electrons in outer shell. Shares with silicon atoms to form a crystal bond of 7 electrons and 1 hole, readily attracting extra electrons.

^^ Phosphorus Atom: 5 electrons in outer shell. Shares with silicon atoms to form a crystal bond of 8, plus one extra electron.

© Electron: Knocked around by energy of sunlight; moves through circuit from N-layer to P-layer.

photovoltaic effect

Cartoon Loto Working Machine

Measuring single-crystalline silicon ingots at the SolarWorld PV plant in Vancouver, British Columbia.

Electrons & Efficiency

One way to think about the process of electron movement is to imagine that the P-layer is a pool filled with electrons and your deck is the N-layer. If a sufficiently strong photon hits one of the electrons in the pool (P-layer), it can kick it up onto the deck (N-layer) where you can catch it and put it to useful work. Ideally, every photon coming into the pool would bump an electron up onto the deck that you could collect and put to use. However, silicon's limitations, along with design challenges, prevent PV cells from being 100% efficient. In reality, most commercially available cells are between 4% and 22% efficient at converting the energy in the photons to useful electricity. Here are several reasons why:

Too Little or Too Much Energy. The light that hits a cell contains photons with a wide range of energies, but a PV cell will only respond to certain energies, or wavelengths. The required level of photon energy to activate an electron is referred to as the band gap. Different types of photovoltaic

Measuring single-crystalline silicon ingots at the SolarWorld PV plant in Vancouver, British Columbia.

wires (traces) attached to the N-layer gives the electrons someplace to go, and they enter a DC circuit, flowing from the negative side of the cell and re-entering the cell through the positive side.

PV modules are made by connecting numerous cells in series, parallel, or series/parallel to achieve useful levels of voltage and current. These cell networks include positive and negative wiring terminals so we can channel the electricity generated to our uses. As long as sunlight is coming in, the electrons will keep flowing and can deliver electrical energy to a load that's connected to the circuit.

Cell Encapsulant

PV Module Anatomy

Densely spaced traces on the back of a PV cell help transfer electrons to the P-layer.

Glass:

Tempered, antiglare

Module Encapsulant:

Ethylene vinyl acetate

Glass:

Tempered, antiglare

Module Encapsulant:

Ethylene vinyl acetate

Cell Encapsulant

Traces: Metallic conductors

Antireflective Coating

Traces: Metallic conductors

Note: Material thicknesses not to scale

Note: Material thicknesses not to scale

Traces: Metallic conductors

Antireflective Coating

N-Layer Silicon:

f Phosphorus doped

P-Layer Silicon:

Boron doped

Traces: Metallic conductors odule Encapsulant:

Ethylene vinyl acetate

Back Sheet:

Polyvinyl fluoride film materials have different band gaps—higher and lower decks, so to speak. Some photons don't have enough energy, and although they bump electrons, they don't give them enough energy to get them up on the "deck." This energy is wasted as heat. The lower the deck (lower band gap), the lower the minimum energy required.

So why can't we choose a material with a really low band gap, so we can use more of the photons? Unfortunately, the band gap also determines the voltage of our solar cell. If it's too low, what we make up in extra current (by absorbing more photons) we lose by having a small voltage (remember that power is voltage times current). If the incoming photon photovoltaic effect is too strong, it bumps the electron up higher than the deck, before it falls back down. In a PV cell, this energy expenditure is also wasted in the form of heat.

To capitalize on the higher energies of some photons, some exotic PV materials have two levels of decks. If a photon has enough energy, it can bump the electron all the way up to a higher deck where it can be collected. Some amorphous PV modules have two or three levels of decks, so if an electron isn't excited enough to get on the highest deck, it might at least end up on a lower one and be used there.

These two effects alone—too little energy and too much energy in incoming photons—account for the loss of about 70% of the radiation energy incident on our cell.

Imperfect Junctions. A second source of inefficiency is that a lot of electrons just roll through slots between the deck boards before you can collect them. A perfect crystal doesn't have any holes—every electron that is collected stays on the deck until it can be collected. However, polycrystalline solar cells have joints between crystals, resulting in an imperfection in the P-N junction—holes in the deck, so to speak, that allow electrons to slip back into the pool before they can be collected.

Even in a single-crystal solar cell, you still can't collect all the electrons. The metal traces that collect electrons in a PV cell are spaced apart, and an electron that ends up too far from it may be lost before it can travel to the nearest trace and be collected.

Amorphous silicon has a similar problem called hydrogen diffusion. Instead of being a solid silicon crystal, it has all kinds of loose hydrogen atoms, which function like a deck full of gaps. Also, electrons in a position to be bumped by photons are fewer and farther between because the hydrogen leaves less silicon to hit. The hydrogen atoms are the reason that amorphous silicon decreases in efficiency over the first few months before stabilizing: Hydrogen in the atmosphere slowly diffuses into the module.

Reflection, Obstruction & Temperature.

Silicon is very reflective, which makes harvesting sunlight challenging, since a cell can't use photons that are reflected. For that reason, an antireflective coating (typically titanium dioxide or silicon nitride) is applied to the top of the cell to reduce reflection losses to less than 5%. This coating is what gives solar cells their blue appearance, instead of gray, as raw silicon would appear. The antireflective coating can be modified to get different colors, such as red, yellow, green, or gray, but these colors are less efficient than dark blue, so you very rarely see PV modules in these other colors. The glass on a module also has a special textured surface to minimize the reflection of sunlight.

PV Cell Particulars

Model-T maker Henry Ford was fond of telling consumers they could have any color car, "so long as it was black." Options in PV module choices used to be as limited, but that's changing. Today, you can choose from three basic types of PV modules: monocrystalline, polycrystalline, and thin-film.

Most of us are familiar with the iridescent-blue faces of monocrystalline and polycrystalline modules. In both cases, fragile razor-thin wafers of silicon are embedded in a rigid frame and protected behind a layer of tempered glass. The difference between the two crystallines lies in the production of the cell. Monocrystalline ingots are extracted from melted silicon and then sawed into thin plates. Polycrystalline cells are created by pouring liquid silicon into blocks that are sawed into plates.

In the thin-film process, a silicon film (or other materials, such as cadmium telluride or copper indium gallium selenide) is deposited on glass or stainless steel, or within a flexible laminate. Although production costs are lower due to lower material costs, the efficiency of thin-film modules is typically about half that of either mono- or polycrystalline cells.

R&D technicians inspect a monocrystalline wafer at a Suntech Power PV plant in China.

R&D technicians inspect a monocrystalline wafer at a Suntech Power PV plant in China.

Because silicon is a semiconductor, it's not nearly as good as a metal for transporting electrical energy. Its internal resistance is fairly high, and high resistance means high losses. To minimize these losses, a cell is covered by a metallic contact grid that shortens the distance that electrons have to travel from one side of the cell to the other while covering only a small part of the cell surface. We could cover the bottom with a metal, allowing for good conduction, but if we completely cover the top too, photons can't get through the opaque conductor and we lose all of our energy. If we put our contacts only at the sides of our cell, the electrons have to travel an extremely long distance (for an electron) to reach the contacts.

Various solutions to this obstruction have been considered, from BP Solar's laser-grooved buried-grid modules that put the collection grid in trenches instead of using flat ribbons on the surface, to placing the metal contacts on the back surface of the cell (as on SunPower modules), to transparent conducting layers that are being used for some amorphous and organic PV materials.

Temperature also affects a cell's efficiency. Typically, for each degree centigrade increase in operating temperature over its rated temperature, a PV cell loses about 0.5% of its specified power. For example, a PV module that experiences temperatures 50°C higher than its rated temperature (which is quite common for rooftop modules) may produce 25% less than its rated power. This happens because the thermal energy is distributed unevenly, with some electrons having enough energy to "go the wrong way"—back across the barrier, where they fall into holes we don't want them to.

The Reality of Efficiency

After all this talk about efficiency, you might be surprised to discover that buying the most efficient module on the market shouldn't be your only goal. When you're talking about energy production, it's watts that we're really after. If a less efficient PV module allows us to get those same watts for less cost, it may be a more cost-efficient choice than a more efficient, but more expensive, module.

If you have limited space on your roof or a small solar window, using more efficient modules can often make sense. But if you have acres of warehouse roof, for example, it may not. It all depends on your particular situation. To optimize your investment, prioritize cost per installed kilowatt-hour, longevity, and efficiency, in that order, if space is not a consideration.

Access

Zeke Yewdall ([email protected]) is chief engineer at Sunflower Solar, a PV design/install company in Boulder, Colorado.

Sam Ley ([email protected]) is a physicist who works at Sunflower Solar, and has extensive experience in science education at museums.

Portions of this article were adapted from Scott Aldous's article, "How Solar Cells Work," courtesy ©2007 HowStuffWorks.com.

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